The present invention relates to an apparatus and a method for preventing the formation of tarnish on silver, copper, or brass objects. In particular, the present invention relates to a construction of such apparatus using carbonaceous adsorbents to significantly reduce or eliminate tarnishing agents from contacting of such objects.
Tarnish is a term which describes the visual change in the lustrous surface of a metal after it has reacted with various atmospheric components. Most frequently, tarnish is associated with the discolorization of objects made of silver; silver alloys, in which silver is the predominant component; copper; or brass, which is an alloy of copper and zinc. The chemical process which causes tarnish is the corrosion of the metal surface through an oxidative process. However, the primary route of oxidation, in many instances, can be caused by materials other than oxygen. It is well known that silver and silver alloys tend to tarnish in the gaseous atmospheres containing hydrogen sulfide or other sulfide compounds, even in small concentrations, particularly in the simultaneous presence of ammonia, resulting in a dull or black finish. Copper reacts with hydrogen sulfide in a similar fashion as silver because both elements have the same outermost electron shell configuration according to the Periodic Table. However, copper is more reactive than silver and after the initial tarnishing by hydrogen sulfide to produce a red or dark brown surface, it can further react with carbon dioxide in the atmosphere to produce a green patina which has been associated with older copper structures such as the Statue of Liberty.
The sources and concentrations of hydrogen sulfide in the atmosphere and indoor air are varied. Man-made sources of hydrogen sulfide include industrial emissions from the burning of fossil fuels, natural gas, chemical plants, poorly ventilated sewage lines, sewage treatment facilities, and farms. Natural sources include geothermal emissions, anaerobic bacterial processes, and salt water bodies. In the indoor environment, gaseous by-products from human digestive processes are a major source. It is, therefore, difficult to place a value on what might be considered an average or typical range for hydrogen sulfide concentrations in the atmosphere or indoor environment. The U.S. Environmental Protection Agency (xe2x80x9cUSEPAxe2x80x9d) has estimated that the average concentration for hydrogen sulfide in the atmosphere is 0.1 to 0.3 parts per billion (xe2x80x9cppbxe2x80x9d) (see USEPA report number EPA/600/8-86/026F, Health Assessment Document for Hydrogen Sulfide, January 1993). It should be clearly noted that this is an average concentration and peak levels may be higher by several hundred fold as is evident in the USEPA report.
The control of hydrogen sulfide also has long been a concern in the area of prevention of corrosion, due to low levels of hydrogen sulfide, of silver and copper electronic contacts in sensitive electronic equipment. The tarnishing and corrosion of silver and copper contacts can result in the premature failure of electronic equipment.
Sulfide films on silver or copper objects can be removed from their surfaces by chemical treatment or polishing. The formation of sulfide films can be prevented by forming on the objects a film of another stable metal, such as rhodium, or of oxide of aluminum, beryllium, technetium, or zirconium. However, all of these methods have disadvantages. Chemical treatment and polishing gradually remove part of the original metal. Accidental scratches in the film of stable metal or oxides expose the fresh silver or copper surface to attack by hydrogen sulfide.
Therefore, it is an object of the present invention to provide an apparatus and a method for preventing or inhibiting the formation of tarnish on objects made of silver, copper, or mixtures thereof. It is a further object of the present invention to provide regenerable activated carbon articles forming interior surfaces or linings of such an apparatus for surrounding such objects. These and other objects of the present invention will become apparent upon a perusal of the present disclosure.
In general, the present invention provides an apparatus comprising flexible adsorbent articles for enclosing objects made of silver, copper, or mixtures thereof significantly reduce or eliminate tarnishing agents from therein and to prevent or inhibit the formation of tarnish on such objects. The flexible articles comprise one or more adsorbents, such as activated carbon, natural or synthetic zeolite, activated alumina, or silica gel. These adsorbents may be incorporated in flexible fibrous matrixes which are capable of being formed into a variety of shapes.
One particularly suitable adsorbent is activated carbon cloth (xe2x80x9cACCxe2x80x9d), which has a faster adsorption kinetics than the common forms of activated carbon, is manufactured in thermal processes from woven or non-woven natural, man-made, or synthetic fibers and which possesses a high surface area for adsorption. The raw material fiber is preferably chosen for its desirable properties. Flexible ACC removably attaches to interior surfaces or linings so that the ACC substantially surrounds the metal objects to be protected.
Commonly encountered tarnishing agents include hydrogen sulfide, sulfur oxides, carbonyl sulfide, carbon disulfide, nitrogen oxides, ozone, halogen gases, ammonia, ammonium salts, and mixtures thereof. The reaction of many of these tarnishing agents on silver, copper, and mixtures thereof may be accelerated in the presence of water vapor. When the adsorptive capacity of the ACC has been exhausted, it may be removed from the apparatus and regenerated for reuse. Regeneration may be achieved by, for example, washing the ACC and heating it to a temperature less than about 350xc2x0 C. The regenerated ACC may then be reinserted or reattached to the apparatus to provide continued protection of the metal objects.